Block Hash Calculation Algorithm

Bitcoin Tutorials - Herong's Tutorial Notes

∟Block Hash Calculation Algorithm

This section describes the Block Hash Calculation Algorithm.

Bitcoin uses the following algorithm to calculate the block hash:

1. Get values from block header fields: version, hashPrevBlock, hashMerkleRoot, time, bits, nonce.
2. Convert field values in little-endian hex notation, if they are displayed in big-endian hex notation as with most Bitcoin tools including Bitcoin Core and blockchain.com.
3. Concatenate header field values into a header string: header = version+hashPrevBlock+hashMerkleRoot+time+bits+nonce.
4. Take the double-SHA256 hash value from the header string: hash = SHA256(SHA256(header)).
5. Return the hash value as the block hash.
6. Convert the hash value in big-endian hex notation, in order to compare it with other Bitcoin tools.

First lets follow the Python example given in the Block hashing algorithm at https://en.bitcoin.it/wiki/Block_hashing_algorithm, which calculates the block hash of Bitcoin Block 125552, https://www.blockchain.com/btc-testnet/block /00000000000000001e8d6829a8a21adc5d38d0a473b144b6765798e61f98bd1d. Note that I had to replace string decode() and encode() with binascii.unhexlify() and binascii.hexlify() on my Python installation.

C:\>python
>>> import hashlib
>>> import binascii

>>> header_hex = ("01000000" +
...  "81cd02ab7e569e8bcd9317e2fe99f2de44d49ab2b8851ba4a308000000000000" +
...  "e320b6c2fffc8d750423db8b1eb942ae710e951ed797f7affc8892b0f1fc122b" +
...  "c7f5d74d" +
...  "f2b9441a" +
...  "42a14695")
>>> header_bin = binascii.unhexlify(header_hex)
>>> hash = hashlib.sha256(hashlib.sha256(header_bin).digest()).digest()

>>> binascii.hexlify(hash)
b'1dbd981fe6985776b644b173a4d0385ddc1aa2a829688d1e0000000000000000'

>>> binascii.hexlify(hash[::-1])
b'00000000000000001e8d6829a8a21adc5d38d0a473b144b6765798e61f98bd1d'

The result matches will with the block hash given in blockchain.com:

Block #125552
BlockHash 00000000000000001e8d6829a8a21adc5d38d0a473b144b6765798e61f98bd1d

Table of Contents

About This Book

Introduction of Bitcoin

Bitcoin-Qt - Bitcoin Core GUI

Bitcoin Mining

Bitcoin Consensus Rules

►Bitcoin Block Data Structure

Data Components of Bitcoin Block

Data Properties of Bitcoin Block

Merkle Root of Bitcoin Block

Calculate Double-SHA256 Hash with Python

Verify Merkle Root of 2 Transactions

Verify Merkle Root of 7 Transactions

Data Structure of Bitcoin Block

"getblock blockhash 0" - Serialized Hex Block Data

►Block Hash Calculation Algorithm

Block Hash Calculation in Python

Calculate Double-SHA256 Hash with Java

Bitcoin Transaction Data Structure

Bitcoin Blockchain APIs

Copay - Bitcoin Wallet

Archived Tutorials

References

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